1. Field of the Invention
The present invention relates generally to an organic electrolyte lithium secondary battery of which negative electrode consists of metallic lithium or lithium alloy and using lithium as an active material, and more particularly, to a separator of the same battery.
2. Description of the Prior Art
The lithium secondary battery of which negative electrode consists of metallic lithium or lithium alloy and using lithium as an active material and organic electrolyte draws attention to the point of a light-weight and compact power source, for the same battery has a higher energy density due to a higher voltage, and shows a better performance at low temperatures than general secondary batteries having an electrolyte of an aqueous solution.
However, active lithium deposited on a negative electrode in charging reacts to and decomposes an organic solvent of electrolyte, and deposited metallic lithium grows to dendrite and reacts to the organic solvent and then forms an insulating layer on the dendrited lithium surface, which results in forming poor-electronic-conductive lithium. R. Selim Bro, J. Electrochem. Soc, 121, 1457 (1974), etc. already reported this problem. This phenomena lowers the charge and discharge efficiency of a negative electrode composed of metallic lithium and lithium alloy and also reduces a battery capacity in the progress of charge and discharge cycles. Further, the dendrited metal lithium penetrates a separator to shortcircuit the battery inside, which shortens a cycle life. Due to these problems, the organic electrolyte lithium secondary battery of which negative electrode consists of metallic lithium or lithium alloy and using lithium as an active material is not yet practically used.
The occurrence of the dendrited lithium correlates with a charge current density, namely, the higher the current density is, the easier occurs dendrited lithium. In general, the lithium secondary batteries thus are devised to have a less charge current density: In order to enlarge the areas of each facing plate of negative and positive electrodes, the thin electrode plates are scrolled with a separator in between. The such electrode plate group and organic electrolyte have been studied to comprise the battery. However, since the plate surface is not smooth enough, a reaction occurs at some particular places, which forms dendrited lithium. The dendrited lithium thus formed on a negative electrode of lithium penetrates through a separator made of microporous membrane, and produces internal shortcircuit, which results in not only damaging battery performance as well as shortening a battery life, but also, in the worst case, generating heat to start burning.
In order to solve the problem of forming dendrited lithium, several intentions were already laid out, for example, Japanese Laid-Open No. Hei 3-129678 bulletin proposes that a wider positive electrode than that of a negative be used, and also, fringe part of a positive electrode be shielded with an insulating material having the same thickness of the positive electrode before facing to the counterpart of a negative electrode. The bulletin of Japanese Laid-Open No. Hei 4-51473 proposes that the fringe part of positive electrode be shielded with an electrically insulating material which is insoluble in electrolyte. Unfortunately, these inventions cannot show a practically satisfied result, for they have such problems as require a complicated manufacturing process, and have less capacity for active material being filled. In addition, an insufficient cohesive strength on the fringe part of positive electrode prevents from avoiding an internal shortcircuit due to the dropping of positive mixture. Further, Japanese Laid-Open No. Hei 1-319250 bulletin proposes that ion-permeable polymer such as polyacrylamid be coated to a separator in order to avoid penetration of fine particle of positive mixture into a separator. However, polyacrylamid is easy to solve in organic electrolyte, and forming a high density polymer-layer results in high internal resistance of the battery. These factors lower the battery performance. The bulletin of Japanese Laid-Open No. Hei 2-162651 proposes that the battery wherein electrode plate groups are wound have electrode plates on which solid polymer electrolyte film is formed. However solid polymer electrolyte film reacts to an active material of negative electrode to become deteriorated, and the same film has a problem in mechanical strength. The U.S. Pat. No. 5,281,491 proposes that multilayer porous sheet of different rheological properties be used as a separator. Yet, it requires complicated process to manufacture the batteries and accompanies a higher price of the separator.
Regarding the polymer electrolyte, the bulletin of Japanese Laid-Open No. Hei 2-291673 proposes that organic electrolyte be applied into it as a plasticizer to have it turned to gel electrolyte so that ion conductivity is dramatically improved.
In order to boost the mechanical strength of polymer electrolyte, it is laid open that polymer electrolyte and a separator are hybridized on the bulletins of Japanese Laid-Open No. Hei 4-36959 and 5-500880. However, when using polymer electrolyte, a positive electrode and negative electrode are not contacted with polymer electrolyte on the interface in between. This problem produces gaps on the interface particularly in charging or discharging because both of the electrode plates expand or shrink, which interferes with a smooth progress of efficient charge or discharge reaction.
The proposed ideas for improvement still have some problems as mentioned above. Indeed the internal shortcircuit due to the dendrited lithium can be avoided, but using an ion-conductive gel electrolyte independently proves that mechanical strength is poor, reliability is low and ionic conductivity remains insufficient at low temperatures. Since these factors lower the battery performances, the proposed ideas are not yet commercialized.